<template>
  <div ref="list" :style="{height}" class="infinite-list-container" @scroll="scrollEvent($event)">
    <div ref="phantom" class="infinite-list-phantom"></div>
    <div ref="content" class="infinite-list">
      <div class="infinite-list-item" ref="items" :id="item._index" :key="item._index" v-for="item in visibleData">
        <slot ref="slot" :item="item.item"></slot>
      </div>
    </div>
  </div>
</template>


<script>

export default {
  name:'VirtualList',
  props: {
    //所有列表数据
    listData:{
      type:Array,
      default:()=>[]
    },
    //预估高度
    estimatedItemSize:{
      type:Number,
      required: true
    },
    //缓冲区比例
    bufferScale:{
      type:Number, 
      default:1
    },
    //容器高度 100px or 50vh
    height:{
      type:String,
      default:'100%'
    }
  },
  computed:{
    _listData(){
      return this.listData.map((item,index)=>{
        return {
          _index:`_${index}`,
          item
        }
      })
    },
    visibleCount(){
      return Math.ceil(this.screenHeight / this.estimatedItemSize);
    },
    aboveCount(){
      return Math.min(this.start,this.bufferScale * this.visibleCount)
    },
    belowCount(){
      return Math.min(this.listData.length - this.end,this.bufferScale * this.visibleCount);
    },
    visibleData(){
      let start = this.start - this.aboveCount;
      let end = this.end + this.belowCount;
      return this._listData.slice(start, end);
    }
  },
  created(){
    this.initPositions();
    window.vm = this;
  },
  mounted() {
    this.screenHeight = this.$el.clientHeight;
    this.start = 0;
    this.end = this.start + this.visibleCount;
  },
  updated(){
    this.$nextTick(function () {
      if(!this.$refs.items || !this.$refs.items.length){
        return ;
      }
      //获取真实元素大小，修改对应的尺寸缓存
      this.updateItemsSize(); 
      //更新列表总高度
      let height = this.positions[this.positions.length - 1].bottom;
      this.$refs.phantom.style.height = height + 'px'
      //更新真实偏移量
      this.setStartOffset();
    })
  },
  data() {
    return {
      //可视区域高度
      screenHeight:0,
      //起始索引
      start:0,
      //结束索引
      end:0,
    };
  },
  methods: {
    initPositions(){
      this.positions = this.listData.map((d,index)=>({
          index,
          height:this.estimatedItemSize,
          top:index * this.estimatedItemSize,
          bottom:(index+1) * this.estimatedItemSize
        })
      );
    },
    //获取列表起始索引
    getStartIndex(scrollTop = 0){
      //二分法查找
      return this.binarySearch(this.positions,scrollTop)
    },
    binarySearch(list,value){
      let start = 0;
      let end = list.length - 1;
      let tempIndex = null;

      while(start <= end){
        let midIndex = parseInt((start + end)/2);
        let midValue = list[midIndex].bottom;
        if(midValue === value){
          return midIndex + 1;
        }else if(midValue < value){
          start = midIndex + 1;
        }else if(midValue > value){
          if(tempIndex === null || tempIndex > midIndex){
            tempIndex = midIndex;
          }
          end = end - 1;
        }
      }
      return tempIndex;
    },
    //获取列表项的当前尺寸
    updateItemsSize(){
      let nodes = this.$refs.items;
      nodes.forEach((node)=>{
        let rect = node.getBoundingClientRect();
        let height = rect.height;
        let index = +node.id.slice(1)
        let oldHeight = this.positions[index].height;
        let dValue = oldHeight - height;
        //存在差值
        if(dValue){
          this.positions[index].bottom = this.positions[index].bottom - dValue;
          this.positions[index].height = height;
          for(let k = index + 1;k<this.positions.length; k++){
            this.positions[k].top = this.positions[k-1].bottom;
            this.positions[k].bottom = this.positions[k].bottom - dValue;
          }
        }
        
      })
    },
    //获取当前的偏移量
    setStartOffset(){
      let startOffset;
      if(this.start >= 1){
        let size = this.positions[this.start].top - (this.positions[this.start - this.aboveCount] ? this.positions[this.start - this.aboveCount].top : 0);
        startOffset = this.positions[this.start - 1].bottom - size;
      }else{
        startOffset = 0;
      }
      this.$refs.content.style.transform = `translate3d(0,${startOffset}px,0)`
    },
    //滚动事件
    scrollEvent() {
      //当前滚动位置
      let scrollTop = this.$refs.list.scrollTop;
      // let startBottom = this.positions[this.start - ]
      //此时的开始索引
      this.start = this.getStartIndex(scrollTop);
      //此时的结束索引
      this.end = this.start + this.visibleCount;
      //此时的偏移量
      this.setStartOffset();
    }
  }
};
</script>


<style scoped>
.infinite-list-container {
  overflow: auto;
  position: relative;
  -webkit-overflow-scrolling: touch;
}

.infinite-list-phantom {
  position: absolute;
  left: 0;
  top: 0;
  right: 0;
  z-index: -1;
}

.infinite-list {
  left: 0;
  right: 0;
  top: 0;
  position: absolute;
}

.infinite-list-item {
  padding: 5px;
  color: #555;
  box-sizing: border-box;
  border-bottom: 1px solid #999;
  /* height:200px; */
}

</style>